The BTH B6 is a directly heated audio output valve from 1924. It is similar to the earlier B4 in characteristics but has a lower consumption heater. Additionally the B6 has two straight filaments that are connected in parallel, the B4 has a V shaped filament.
The envelope carries the type three BBC stamp dating this exhibit to between 1924 and 1927.
The electrodes are visible from only one angle due to the silvering inside the envelope. The grid construction is clear but it is difficult to determine the filament details.
The balloon envelope is 50 mm in diameter, and excluding the B7 base pins is 97 mm tall.
References: 1003 & 1004. Type B6 was first introduced in 1924.